Characterization of dynamic crossovers in bulk liquid water by molecular dynamics simulations

Dynamic cross-overs in bulk water are important for the study of glass-forming liquids and implicated in many phenomena of biological interest. Here we use classical molecular dynamics (MD) simulations to characterize the interplay between dynamic crossovers and microscopic behaviour of bulk liquid water in the temperature range 180 – 350 K. In particular, we focused to the dynamic crossover, which is involved in favouring the unfolding of proteins, found experimentally at T* ~ 315 ± 5 K [1,2]. Computationally, a cross-over from Vogel-Fulcher-Tamman to linear trend for increasing temperature was detected from the Arrhenius plots of dynamic quantities, namely of the inverse diffusion coefficient and of the rotational constant and from the onset of a further heterogeneity in the rotational relaxation [4], but we propose some other possible statistical tools. We also verified that for the coefficient of thermal expansion AlphaP(T,P) the isobaric AlphaP (T) curves cross at about the same T* as in the experiment. We used two different potential models for water: TIP4P-Ew and a recently proposed model (OPC) [3]. OPC was developed by Onufriev et al. through the optimization of the distribution of point charges to best describe the “electrostatics” of the water molecule, which reproduces a comprehensive set of bulk properties significantly more accurately than commonly used rigid models. Simulations predict T* ~ 285 ± 5 K for the TIP4P-Ew models, but T* ~ 309 ± 5 K for the OPC model. The lifetimes of water hydrogen bonds and of the nearest neighbours were evaluated and were found to cross near T*, where the lifetimes are about 1 ps. For T < T*, hydrogen bonds persist longer than nearest neighbours, suggesting for T < T* a prevalence of the hydrogen bonding network in water structure, while for T > T* water behaves more like a simple liquid. The fact that T* falls within the biologically relevant temperature range is a strong motivation for further analysis of the phenomenon and its possible consequences for biomolecular systems. [1] F. Mallamace, C. Corsaro, H. E. Stanley. Sci. Rep. 2, 993 (2012). [2] F. Mallamace, C. Corsaro, D. Mallamace, S. Vasi, C. Vasi, H. E. Stanley, J. Chem. Phys. 141, 18C504 (2014) [3] S. Izadi, R. Anandakrishnan, A. V. Onufriev, J Chem, Phys. Lett. 5, 3863 (2014). [4] P. Demontis, J. Gulín-González, M. Masia, M. Sant, G. B. Suffritti, J. Chem. Phys. 142, 244507 (2015)